The present invention relates to tunable ring lasers and more particularly to an improved way of tuning a ring laser, such as a dye laser.
Tipping an inserted glass plate about a small range of angles near Brewster's angle has become a standard means to change the length of optical path in a single-frequency dye laser cavity to produce a frequency scan. For a -2.degree. to +2.degree. tip range away from Brewster's angle of incidence (55.6.degree. for a silica plate) the Fresnel equation gives less than 0.1% maximum reflection loss, which is acceptable for an intracavity element in a dye laser, and a 30 .mu.m path length change for a silica plate 1 mm thick. This path length change is enough for a scan of 100-cavity mode spacings for a laser operating at 600 .mu.m wavelength. Not only is this a long scan, but the scan is very linear with tip angle. The first use of this technique to scan a dye laser was made by Schuda, Hercher and Stroud, Appl. Phys. Lett. 22, Vol. 22, Apr. 15, 1973, pp. 360-362.
An alternative method of changing the cavity path length is to mount an end mirror on a piezoelectric element and drive this assembly with a high voltage ramp. This technique is typically limited to one-tenth the range afforded by a tipping Brewster plate and has five times the non-linearity.
Traveling-wave CW ring dye lasers are capable of several-watt single-frequency outputs because they may be pumped with the full power of available ion lasers. In contrast, an input power limit exists in a standing-wave dye laser due to the regions of unsaturated gain in the pumped volume of the dye jet at the nodes of the standing wave. It has been shown that the fraction of the total volume that the unused portion represents, decreases as the dye beam intensity increases. The drop in volume, however is less rapid than the linear rise in pump power. Thus, a mode at a second frequency, which has antinodes where the first mode has nodes, must eventually reach threshold and oscillate as the pump level is increased in the standing wave case. This limit does not exist in a ring laser, and typically a ring can be pumped four times harder than a standing wave laser.
The ring laser cavity typically employs a four-mirror, figure-eight configuration to keep the fold angles small, allowing astigmatic compensation with a Brewster plate of reasonable thickness. But in a ring laser, a conventional tipping Brewster plate has a major disadvantage in that the lateral beam displacement which accompanies the path length change misaligns the optical resonator ring, and produces an unacceptable power modulation of the dye laser over the frequency scan. For a 30 .mu.m path length change for the tipping Brewster plate, the lateral beam displacement is 50 .mu.m. The spot size of the beam in the dye cavity is 500 .mu.m, and the optical loss produced by this displacement may be as large as 10%.
In a conventional three-mirror "linear cavity" dye cavity, the beam also experiences lateral displacement but since it is incident only on a flat mirror, the beam returns upon itself. Thus, the displacement of the return beam is "undone" in the second traversal of the tipping plate, and there is no displacement of the beam at the curved mirrors or the jet.